Prior research in the crack healing of glass has indicated that fracture is a partially reversible process and that cracks in glass surfaces can be propagated or healed by controlling the applied load. The reason for the reversible nature of the bonding process appears to reside primarily in the characteristics of the crack tip. It is the thought that atomic forces acting across the crack tip tend to pull crack surfaces into realignment, thus re-establishing contact over the entire crack plane. Although crack healing in single crystals has been reported, it is an infrequently-observed phenomenon.
Some evidence of crack closure in glass has been reported. Most commonly when cracks do occur in glass, their healing or closure does not fully result or is incomplete due to films absorbed on the crack surfaces. With regard to strength recovery, it is occasionally possible to obtain 80% of the original strength, depending upon conditions of fracture and recovery.
In some cases, cracks in soda-lime glass samples have been found to close spontaneously. About 80% strength was recovered in cracks formed by mechanical shock while about 20% was recovered in cracks which closed after being held open for several minutes. The high strength recovery in mechanically-shocked samples is considered traceable to the very active surface formed by fracture. When the surface is allowed to adsorb oxygen or water vapor, the surface activity is decreased and the resultant healing is less complete. Such studies have been reported in the paper entitled, "Crack Healing In Glass", by S. M. Weiderhorn and P. R. Townsend presented at the 72nd Annual Meeting, the American Ceramic Society, May 6, 1970 (Glass Division, No. 33-G-70).
It is also possible to strengthen glass by fire polishing or fire glazing whch is carried out by directing flames onto the glass surface to heat soften or melt the same. Such procedure may be useful in working certain types of glass products, but in the case of glass containers and other precisely-formed glass articles, very special care must be taken not to deform localized regions of the glass sidewalls or the neck and finish portions which deformation can result in other defects.